The recommended dietary allowance for calcium for the female adolescent is an extrapolated figure. The calcium intake for maximal accretion is unknown. Meeting calcium requirements for maximal bone growth may be the most crucial in this age group to lower the risk of developing osteoporosis in later years. The primary aim of the proposed study is to establish calcium requirements in adolescent girls between the ages of 12 and 17. Calcium balance and kinetics will be determined in l0 girls aged 15 to 17 who were previously studied at age 12 to 14. The calcium intake required to achieve maximal calcium accretion will be determined in 25 girls aged 12-14 measured twice over 9 intake levels between 800-1860 mg/day. A secondary aim is to establish the interrelationship between calcium requirements and bone turnover, bone density, and indices of calcium homeostasis in order to identify predictors of calcium requirements in the general population. Stable isotopes of calcium will be administered orally and by intravenous injection after a one week equilibrium period during a three week metabolic camp. Stable isotopes will be analyzed by high resolution fast atom bombardment mass spectroscopy. Analysis of complete urine and fecal collections and periodic blood samples will provide data for multicompartmental and stochastic analysis of calcium metabolism. Bone mass and total body calcium determined by dual energy X-ray absorptiometry, hormone levels (PTH and vitamin D metabolites) and biochemical markers of bone turnover will be determined each year to correlate accumulation of bone mass with the rate of bone remodeling. Bone formation will be assessed by serum osteocalcin and bone specific alkaline phosphatase and bone resorption by urinary hydroxyproline and collagen crosslinks. Correlations between calcium retention/kinetics and hormone levels, biochemical markers of bone turnover and bone mass, and total body calcium will be determined.